Acrylic compositions and articles made from them are well known for their clarity, sparkling color, surface gloss and weather resistance. They are also well known for their low-impact strength or brittleness. It has become standard practice to incorporate toughening agents or impact modifiers in acrylic formulations to make them more durable. The incorporation of such functional additives can, however, detract from some of the desirable characteristics of the acrylics, for example the addition of large quantities of rubbery polymers can lower the softening point, contribute to haze, and affect the weatherability.
One type of polymeric impact modifier is sequentially polymerized acrylic compositions comprising a core, an intermediate and a final stage, although there can be some intermingling of the “layers”. In general, these are known as “core/shell” polymers.
There are many factors which determine the efficiency and utility of any specific acrylic core/shell impact modifier. These include the matrix in which it is used or the substrate to which the matrix it is adhered whether alone or as part of a capstock composition. Also highly influent are the particle size and distribution of the modifier, number of layers, the thickness and construction of each layer, the monomer composition of each layer, the type and degree of crosslinking of each layer, the type and degree of grafting, as well as the concentration of the sequentially polymerized core/shell impact modifier in the matrix or primary polymer. The matrix polymer or primary polymer as defined herein is the polymer which forms the bulk of the articles, such as acrylic sheet, or an extruded capstock. One such core/shell impact modified acrylic composition is described in US 2003-0216510.
One way to solve the problems associated with an impact modified layer exposed to the environment is to form a multi-layer structure having an impact modified acrylic layer covered with an acrylic layer containing no impact modifier. Such a structure is described in U.S. Pat. No. 6,444,298.
Some issues found with present multi-layer films are surface haze and delamination from substrates.
Surprisingly it has been found that a multi-layer acrylic composition, having an outer layer containing small hard-core impact modifiers, and an inner layer having larger size impact modifers presents an excellent balance of weatherability, chemical resistance, abrasion resistance, and gloss after thermoforming.